1. The Field of the Invention
The invention relates to an arrangement and a method for preventing or reducing the de-polarization of linear-polarized light during the transmission of light through an optical medium, particularly through crystals. The invention also relates to the use of such an arrangement.
2. The Description of the Related Art
Linear-polarized light is an electromagnetic transversely propagating wave that vibrates only in a plane transversal to the direction of propagation. If additionally the light is also coherent, all waves propagate synchronously. The light beam thus consists of a bundle of such individual, equally oriented waves. Linear-polarized light finds use in a wide area of technology, for example in the production of computer chips or flat-panel displays and TFT monitors. When linear-polarized light impinges on a polarizing filter which transmits only light vibrating in a certain plane, then depending on its position relative to the plane of vibration of a polarized light beam said filter is either permeable in unhindered manner or impermeable (vertical and horizontal setting). The generation of polarized light has been known for a very long time and is brought about, for example, by means of a birefringent crystal such as calcite (CaCO3).
Another possible method for generating polarized light or a polarized radiation involves the reflection from a surface at a Brewster angle. If the light or the radiation beam is reflected at a Brewster angle and then refracted, the reflected beam is polarized entirely in the direction perpendicular to the plane of refraction or of reflection (s-polarization) and the transmitted beam contains without any reflection losses the entire radiation that is polarized parallel to the refraction plane or to the reflection plane (p-polarization) and some of the s-polarized radiation.
A device for utilizing this effect is, for example, a Nicols prism or a Brewster prism which redirects p-polarized radiation to the extent of 90° without losses. Other examples are laser windows disposed at a Brewster angle or prisms disposed at a Brewster angle which allow transmission of p-polarized radiation without reflection losses and only minor transmission for s-polarized radiation and can be used to create polarized light particularly, for example, for a laser, as described in U.S. Pat. No. 6,556,613. Another device for generating such polarized radiation is, for example, a laser. Laser radiation, particularly pulsating laser radiation, is characterized by the fact that, depending on the kind and arrangement of such a device, it is possible to generate not only coherent laser light but also polarized or unpolarized laser light. This, for example in an excimer laser for micro-lithography, occurs in that the optical components (for example the prisms and exit windows) are disposed at a Brewster angle to outcouple a polarization direction.
It has been known for a long time, however, that when linear-polarized laser light passes through an optical medium, for example through a lens or a prism etc, it loses part of its polarization, namely it becomes depolarized. Linear-polarized light thus gives rise to elliptically polarized light, or the direction of vibration is modified. This effect in itself is known.
In TFT technology the background light first passes through a vertically disposed polarizing filter and then through a variable polarizing filter the polarization direction of which can be changed from vertical to horizontal by applying an electric voltage. In this manner, the background light (horizontal orientation) can be fully blocked or (vertical orientation) fully transmitted. If in such a case, however, the polarized light is depolarized between the filters, the light can no longer be completely extinguished or it can no longer be fully transmitted through the filtering system, namely both the luminosity and the darkness undergo a loss in intensity.
Another major technical problem is the depolarization of polarized laser light. In the production of semiconductors and other electronic components in particular, such an effect leads to imaging inaccuracies. Thus, J. H. Burnett in Phys. Rev. B, vol. 64 (2001), 241102 (R), 1-4, has already described the intrinsic birefringence in calcium fluoride and barium fluoride crystals used, for example, for the afore-said production of electronic components. In other words, in such systems with purely cubic symmetry it is theoretically not to be expected that birefringence would occur, namely that direction-dependent differences in optical properties such as absorption and refractive index would arise. To pre-vent such depolarization-induced illumination problems in precision UV technology, it has been proposed (WO 02/093201 A2) to use differently oriented lens systems, namely a combination of two {111}-(perpendicular) oriented lenses that can be rotated through 60° relative to one another and two {100}-oriented lenses rotated through 45° relative to one another.
M. Letz et al. also describe in “Proceedings of SPIE”, vol. 4691, pp. 1761-1768 (2002) the in itself surprising strain birefringence in purely cubic crystal systems such as CaF2. They describe the correlation between the optical anisotropy and the dispersion of the gamma exciton. This correlation was demonstrated experimentally with the aid of synchrotron scattering (M. Letz et al., Phys. Rev. B, 67, 233101 (2003).